Processes for the protection of magnesium and magnesium base alloys against corrosion



UNI ED. STATE Patented Oct. 23, 1945 w MAGNESIUM"-AND='MAGNESIUM'BASEAL- 7 :LOYS AoAINsrcoRnosmN l Y .Charles' James Buslirdd', Prestwi ch,.Manchester, F England, assignor to Magnesium ElektronLimited, London,England, a company of Great Britain No Drawing. Application April 26,1943, Serial No. 484,602. In Great Britain May 28, 1942 2 Claims. Thisinvention relates-to processes for proagalnst corrosion by immersion ina solution which reacts chemically with the metal to produce aprotective film on the surface of the metal.

chromate bath, and the cold acid chromate bath,

tection of magnesium and magnesium base alloys comprising the subjectrespectively of British Patents 353,415 and 287,450 and described in thehandbook "Elektron Magnesium Alloys, 1937 pages 91-951. The 30 minutebath whilst giving "good surface finishes has the disadvantage oflengthy period of treatment and necessity for maintaining the solutionnear to boiling point.

Moreover this hot bath is not so well suited to magnesium base alloyscontaining substantially only manganese as 'an alloying constituent, asto. those alloys which contain aluminium. Alloys of the former typehowever (e; g. Elektron registered Trade-Mark AM503' alloy) are howeverwidely used for the production of sheets for manufacture into weldedarticles such as fuel tanks.

. The problem of treating these tanks therefore cannot well be solved bythe use of the hot bath.

The acid chromate treatment however involves a cold solution andnormally a very short period of immersion but the solution is stronglyacid and-- therefore removes a small but appreciable amount l of themetal (of the order of 0.00025" reduction in. thickness from eachsurface) during treatment and is therefore not so suitable for use onpartsmachined to fine tolerances. In some nitric acid from 5 up to partsto 100 parts of water the weight loss for a 20 second immersionincreased from 14.5 mgs. per square dcm. of exposed surface up to 67, agraph representing this increase being substantially a straight linebetween these two figures. Similar examples with 40 second and 60 secondimmersions gave weight loss increases from 22 to 99.5 and 30.5 to 133.5respectively. The samples were then immersed for 48 hours in a 3% sodiumchloride solution, the further loss in weight caused by corrosion beingthen ascertained. The result showed that the corrosion weight lossdecreased from .40 mgs. per square dcm. per day with 20 parts acid downto 30.5, with 15 parts acid, and to 22.5 with 10 parts acid, and aftersome further reduction in weight loss with further reduced acid contentit was found that still further reduction of acid content resulted in anincrease in the corrosion the metal the acid content of a new bathshould circumstances for example when treating very large fuel tanks itis diflicult to avoid immersion of the work for abnormally long periods,

e.g. 1 minute, with consequential increase of 1 dimensional reductions.I have made a series of experiments with a view to overcoming thisdifiiculty and as a result I have ascertained that if the acid contentof the solution is reduced considerably below the amount heretoforerecommended not only is the rate of metal dissolution reduced to anunexpectedly high extent but an unexpected increase is obtained in theprotective value against corrosion afforded by the film produced.

The composition of the bath as heretofore recommended is 15 parts byweight of potassium or sodium dichromate with 20 to 25 parts by volumeof concentrated nitric acid in 100 parts of 'coldwater. According tocertain tests using specimens of magnesium alloy containing only about1.5% manganese as alloying constituent.

and solutions containing varying amounts of preferably-not be as low as5 parts but should be preferably at'least 6 or 7 parts.

In use careful check should be-made on the acid content of the solutionexcept where a definite area of metal is treated for a predeterminedtime by any given volume of the solution.

According to the present invention therefore magnesium or magnesiumalloys are treated in a solution containing a bichromate together withfrom 5 (or preferably 6 or '7) parts of concentrated nitric acid up to15 (or preferably not more than 10 or 12 in 100 parts of water. Thebichromate may be sodium or potassium bichromate and 15 parts (or say 10to 20) of this extrusions or other wrought form. This alloy may for thepurpose of the present invention be defined as containing 0.5-2.5%manganese and

